In recent years, the application of lithium ion batteries to driving batteries for motorcycles and electric cars, and further to industrial batteries combined with primary power sources, such as solar batteries and wind power generation, in addition to small size equipment applications, such as cellular phones and notebook computers, has been promoted. In such large size applications, the cell size and number of cells that are laminated increase significantly, and the battery capacity also increases incomparably, compared with conventional small size applications, and therefore, their safety and cost have taken on increased important. From such a viewpoint, applying lithium manganese complex oxides to positive electrode materials has been studied.
Lithium manganese complex oxides are known as positive electrode materials having high safety because the oxygen desorption start temperature during charge is higher than that of layered lithium nickelate and lithium cobaltate. In addition, manganese, a main component, is more abundant in terms of resources and is also less costly than nickel and cobalt, and therefore, the lithium manganese complex oxides have great merits particularly in large size applications. Further, in the lithium manganese complex oxides, by replacing part of manganese in the crystal structure with other transition metals, such as nickel, the discharge potential increases to the 4.5 to 5 V range (vs. Li/Li+), and higher energy density can be achieved as the so-called 5 V class positive electrodes.
However, it is indicated that in lithium ion batteries using lithium manganese complex oxides, manganese is easily dissolved in high temperature environments, and problems include storage deterioration and cycle deterioration at high temperature. This is considered to be because water that present in the electrolytic solution, as an impurity, reacts with LiPF6 that is added as a supporting salt to produce HF, a strong acid, and this HF reacts with the lithium manganese complex oxide to thereby dissolve the manganese. The manganese dissolved in the electrolytic solution is deposited on the separator and the negative electrode, and therefore, it may become resistant to lithium ion diffusion, break the film (SEI) on the negative electrode, and further become a catalyst decomposing the electrolytic solution on the negative electrode. Therefore, it is thought that when manganese is dissolved, the battery characteristics deteriorate.
In order to inhibit the dissolution of manganese in the electrolytic solution, Patent Literature 1 discloses a method for modifying the surface of a lithium manganese complex oxide with a bismuth compound.
In addition, Patent Literature 2 discloses a lithium manganese complex oxide on the surface of which an Al2O3 layer obtained from an aluminum alkoxide solution is formed.